Temperature after 1km to 120 degrees - thermostat ?

Hi,

for a few days I have the following problem - drive off, after about 1km the temperature indicator explodes formally up to direction 120°, then shortly before Exitus the indicator goes down at once again to 0 and stays there. Since my 8er was no longer really warm (lt. Display) I type on it, that the thermostat now completely spins.

Have you had this before and should I change the thermonstat or does that rather point to something else???

Thank you for your answers

Greetings
Marc

Oh, yeah, 4.2 ABH from 93.